The present invention concerns photographic cameras provided with a distance-measuring arrangement and a visual indication in the viewfinder of the state of focus of the exposure objective relative to the subject to be shot, such distance-measuring arrangement being of the type comprising means for splitting the incoming image light beam into two component beams, with a photosensitive detector arrangement provided with an electronic evaluating circuit operative for measuring the component beams with respect to symmetry relative to a plane equivalent to the film plane, and provided also with a manual adjuster for adjusting the state of focus of the camera.
Such cameras are found in widespread use for both still and motion-picture photography. The distance or state-of-focus indicating system in the camera viewfinder comprises a calibrated scale, a mixed-image rangefinder or in the case of reflex cameras a matte viewing screen provided with a microprism array, or may be of the split-image type which indicates state of fucus by the registration or non-registration of the edges of two halves of the viewfinder image, and so forth.
These indicating systems are in general coupled to the focussing system for the exposure objective in such a manner that when the user manually moves the focus adjuster, for example by turning an adjuster ring surrounding the exposure objective, this serves to both change the state of focus of the exposure objective and correspondingly alter the state-of-focus indication in the viewfinder.
With these known indicating systems, it is in general not possible for the user to know whether the focal point of the incoming image light beam is located ahead of or behind the exposure plane, or ahead of or behind a measuring plane optically equivalent to the exposure plane. Accordingly, when the objective is out of focus, the user, often as not, begins to correct this by turning the focussing ring in the wrong direction, followed by the correct direction when he sees no improvement; and then, wishing to improve the state of focus somewhat further, is again ignorant of the direction in which to turn the adjuster ring; and so forth, so that in the course of focussing the camera the user is repeatedly changing the direction in which he is moving the manual focus adjuster.